Toxoplasma gondii is the most common cause of retinochoroiditis in the world. Ocular toxoplasmosis relapses despite the use of antibiotics and remains an important cause of loss of visual acuity especially in children with congenital infection as well as the elderly and the immunosuppressed. Unfortunately, our understanding of the mechanisms of protection against ocular toxoplasmosis is fragmentary. Macrophages, microglia and T cells are the major components of the cell infiltrates in ocular toxoplasmosis. An important event in the interaction between these cells is the stimulation of CD40 expressed on macrophages/microglia. We identified a new paradigm by which the immune system through CD40 kills T. gondii. CD40 induces toxoplasmacidal activity in macrophages and retinal microglia. This is dependent on autophagy, a process that targets the parasite to lysosomal degradation. This finding is likely key to ocular toxoplasmosis because in vivo control of this disease requires CD40. Thus, studying the regulation of CD40-induced autophagy will likely identify new molecular targets for eradication of T. gondii and for therapy of ocular toxoplasmosis that will hopefully prevent relapse of this disease. The objective of this application is to understand how CD40 signaling in macrophages and retinal microglia controls ocular toxoplasmosis and how T. gondii subverts these signals. The central hypothesis for the proposed research is that, CD40 mediates resistance against toxoplasmosis in the eye through protein kinase activation and induction of autophagy. In contrast, T. gondii and cytokines triggered by the parasite use a strategy to impair the signaling needed for optimal activation of autophagy allowing the parasite to evade eradication. This hypothesis will be tested using immunochemical studies as well as genetic approaches that block specific vesicular trafficking molecules. In the first specific aim we will determine if autophagy and protein kinase activation induced by CD40 mediate resistance to ocular toxoplasmosis. In the second aim, we will identify the intracellular events by which cytokines prevent optimal induction of autophagy and test whether inhibition of these events improves control of ocular toxoplasmosis. The proposed work may lead to new strategies to eradicate T. gondii and treat ocular toxoplasmosis based on modulation autophagy and/or CD40 signaling.